Geoscience Reference
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5.8.2.3 Interpretation pitfalls
A number of responses need to be accounted for when
interpreting DHEM data. They relate to conductive over-
burden, conductive host rocks, current channelling and
induced polarisation. These all apply to EM data in general
and are described in Section 5.7.6 .
In DHEM the response of an overburden layer resem-
bles a long-wavelength regional response decreasing in
amplitude downhole, and upon which are superimposed
the shorter-wavelength target responses ( Fig. 5.94 ) . These
can be separated using filtering techniques (see Section
2.9.2 ) . The effects of variations in the near-surface con-
ductivity depend on their coupling with the primary field.
They will vary with the different loop locations, and from
drillhole to drillhole.
The response of current channelling (see Section 5.7.2.4 )
can be strong in DHEM as the downhole probe can be in
close proximity to the current channelling paths.
Hughes and Ravenhurst ( 1996 ) describe the DHEM
surveys. Initially, only axial component data were recorded
in drillhole SH20 using five transmitter loops ( Fig. 5.95b ) .
The dB/dt data are shown for the later delay times, chan-
nels 10 to 15 (3.5 ms after pulse turn-off), and show off-
hole responses from two conductors. Note from Fig. 5.95c
how the responses are very weak from the northern loop,
indicating that this loop is poorly coupled to the conduct-
ors. The anomaly at about 75 m depth coincides with the
known
disseminated mineralisation; the anomaly
is probably due to the conductivity of continuous stringer
or massive mineralisation further south in the lens. The
consistent polarity of the southern, central and western
loop responses, and the reverse polarity from the eastern
loop, suggests that the conductor lies to the west of the
eastern transmitter loop. This interpretation is consistent
with the known geology.
The deeper response, at about 225 m depth, did not
coincide with known mineralisation, but is possibly due
to the along-strike extension of
'
zone 1
'
mineralisation
intersected more than 100 m to the south of SH20. The
largest responses are from the central and southern loops,
suggesting that the conductor is located south of the drill-
hole. The response from the eastern loop is weaker than
that from the western and central loops, suggesting that the
conductor is nearer these latter two loops. However, the
opposite polarity of the response from the western loop
suggests that the conductor is located between the western
and central loops.
The response from the deeper conductor was modelled
using a current filament model (see Modelling EM data in
Section 5.7.5.3 ) , but the data allowed the filament to be
located either north or south, or above or below the drill-
hole
'
zone 4
'
5.8.3 Examples of DHEM responses from
mineral deposits
DHEM is widely used in mine- and prospect-scale explor-
ation for massive sulphide mineralisation, especially nickel
sulphides. Good examples include those presented by Jack-
son et al.( 1996 ) and King ( 1996 ).
5.8.3.1 Balcooma volcanogenic massive sulphide
The use of TD-DHEM during the exploration of a massive
sulphide body is demonstrated in this example. It illus-
trates the nature of the axial and three-component DHEM
impulse response of multiple conductors, shows the bene-
fits of using multiple transmitter loops and measuring
three-component data, and demonstrates the analysis
required to interpret the data in terms of the location and
geometry of the conductors.
The Balcooma volcanogenic massive sulphide is located
in northern Queensland, Australia (Huston and Taylor,
1990 ) . It is a polymetallic deposit and is most signi cant
for its copper content. An outcropping gossan led to its
discovery. Host rocks are metasedimentary and metavol-
canic rocks (amphibolite facies), including schistose
metagreywackes, pelites and tuffs. Mineralisation occurs as
four separate lenses, referred to as zones 1 to 4, containing
massive, disseminated and stringer sulphides ( Fig. 5.95a ).
The zones are elongate ellipses striking to the northeast,
parallel to the local strike, and plunging at 15 to 25ยบ degrees
towards the southeast.
. This indicates that, for each loop, the
orientation of the current system is being controlled by the
direction of the primary field through the conductor
rather than always being constrained by the plane of a
plate-like conductor. This suggests that the conductive
mineralisation in this zone is a thick body rather than a
thin plate.
To better constrain the location and orientation of the
conductor, cross-component data were recorded to supple-
ment the existing axial-component data. Drillhole SH20
was surveyed using only the central transmitter loop. With
the aid of computed model responses similar to those
shown in Fig. 5.94 , the three-component data ( Fig. 5.95d )
suggest that the conductor is above and mostly to the south
of the drillhole. The peak response of the A-component,
'
for most loops
'
 
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